6.1.  Content-Transfer-Encoding Syntax

   The Content-Transfer-Encoding field's value is a single token
   specifying the type of encoding, as enumerated below.  Formally:

     encoding := "Content-Transfer-Encoding" ":" mechanism

     mechanism := "7bit" / "8bit" / "binary" /
                  "quoted-printable" / "base64" /
                  ietf-token / x-token